1. Field of the Invention
The present invention relates to a method for forming a reflective metal surface and, in particular, to a silicon interconnect passivation and metallization process designed to maximize reflectance.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These conventional high density projection-type color display devices typically include a light source which emits white light. Dichroic mirrors separate the white light into its corresponding red, green and blue (RGB) bands of light. Each of these colored bands of light is then directed toward a corresponding liquid crystal light valve which, depending upon the image to be projected, either selectively permits or prevents transmission of light therethrough. Those RGB bands of light which are permitted to be transmitted through the light valves are then combined by dichroic mirrors or a prism. A projection lens then magnifies and projects the image onto a projection screen.
FIG. 1 illustrates a conventional LCD projection-type imaging system 100. Imaging system 100 includes a light source 101. White light is emitted from light source 101. Once the light hits the prism 103, the light is separated into its red, green and blue colored bands of light by dichroic filter coatings. Colored light is directed toward liquid crystal display (LCD) light valves 105. When reflected off light valve 105, the colored light waves travel back through the prism and through projection lens 107. Lens 107 magnifies and projects the synthesized color image onto projection screen 109.
FIG. 2 illustrates a cross-sectional view of adjacent pixel cell structures that form a portion of a conventional light valve. Portion 200 of the conventional light valve includes a glass top plate 202 bonded to an interconnect structure 204 by a sealing member (not shown). The sealing member serves to enclose a display area and to separate glass plate 202 from interconnect 204 by a predetermined minute distance. Thus, the light valve has an inner cavity 206 defined by the glass plate 202 and interconnect 204. Liquid crystal material 211, such as polymer dispersed liquid crystal (PDLC), is sealed in inner cavity 206.
In a reflective mode display technology, an image is generated by creating regions within the light valve having differing contrast. This contrast is created by the state of the liquid crystal material above the reflective surface, which in turn regulates the amount of light passing from the ambient to the reflective surface.
During operation of the light valve shown in FIG. 2, selective application of voltage to pixel electrodes 212a and 212b from underlying capacitor structures 218a and 218b through metallization 222 and 224 and via 240 switches pixel cells 210a and 210b on and off. Voltage applied to pixel electrodes 212a and 212b varies the direction of orientation of the liquid crystal material over the the pixel electrode. A change in the direction of orientation of the liquid crystal material at the pixel electrode changes the optical characteristics of the light traveling through the liquid crystal.
If the light valve contains twisted nematic crystal, light passes through the light valve unchanged where no voltage is applied to the pixel electrode, and the light is polarized if a voltage is applied to the pixel electrode. If the light valve contains PDLC, light passes through the light valve unchanged where a voltage is applied to the pixel electrode, and light is scattered if no voltage is applied to the pixel electrode.
One key attribute of light valve performance is the amount of light reflected by the pixel cell. The degree of reflectance of the pixel cell in turn affects other system attributes such as contrast ratio, pixel coherence and brightness efficiency. One approach to enhancing the performance of any reflective mode light valve is to increase the reflectance of the mirror toward the ideal.
In examining FIG. 2, it is apparent that pixel electrodes 212a and 212b will serve as the reflective surface of the light valve. Moreover, the highest (third) intermetal dielectric layer 228 serves as the substrate for the reflective pixel electrodes 212a and 212b. Therefore, the reflectance of the light value is dependent in large measure on the processing steps which follow formation of the highest intermetal dielectric layer 228 and all subsequent layers.
FIGS. 3A-3J illustrate cross-sectional views of the conventional processing steps affecting pixel cell reflectance during formation of adjacent pixel cell electrodes. FIG. 3A illustrates the formation of highest intermetal dielectric layer 328 upon lower metallization layer 324.
FIG. 3B illustrates leveling of the surface of the highest intermetal dielectric layer 328 using the process of planarization by SOG formation and etchback.
FIG. 3C illustrates the patterning of a photoresist mask 330 over the planarized surface of highest intermetal dielectric 328, followed by etching in unmasked areas to create vias 340.
FIG. 3D illustrates formation of a liner layer 342 within vias 340, followed by the formation of a layer of Tungsten 344 over the highest intermetal dielectric 328, filling vias 340.
FIG. 3E illustrates removal of tungsten layer 344 outside of the vias. This step can be accomplished by straight CMP, or alternatively by etchback followed by CMP.
FIG. 3F illustrates formation of the pixel adhesion underlayer 346, typically formed from Ti/TiN. This Ti/TiN layer 346 provides an adhesion surface for the AlCu and thereby prevents degradation of reflectance due to roughness occurring during subsequent thermal exposure. The potential contribution of the pixel adhesion layer to loss of reflectance is described in greater detail in Section 4 of the detailed description of the invention.
FIG. 3G shows formation of the pixel electrode layer 312 on top of pixel adhesion underlayer 346. Pixel electrode layer 312 is conventionally formed by depositing an Al/Cu mixture at approximately 400.degree. C.
FIG. 3H illustrates patterning of a photoresist mask 350 on top of pixel electrode layer 312, followed by etching of unmasked regions of the pixel electrode layer and the pixel adhesion layer 346 to form discrete pixel electrodes 312a and 312b.
FIG. 3I illustrates removal of patterned photoresist mask 350 from the surface of pixel electrodes 312a and 312b to complete formation of reflective pixel electrodes 312a and 312b. Stripping of photoresist mask 350 is conventionally accomplished utilizing a 1) plasma ash, 2) solvent strip, and 3) plasma ash, sequence.
FIG. 3J illustrates formation of a passivation layer 352 on top of the reflective pixel electrodes 312a and 312b. This passivation layer 352 (typically silicon dioxide) is deposited at around 400.degree. C. and protects the surface of the pixel electrodes 312a and 312b.
The conventional process flow depicted above in FIGS. 3A-3J enables creation of a reflective pixel cell. Unfortunately however, several steps of the conventional process inevitably degrade the reflectance of the pixel electrode.
First, the SOG planarization step shown in FIG. 3B generally does not result in an intermetal dielectric layer having a surface that is sufficiently smooth to promote optimum reflectance of the reflective pixel electrode layer to be formed above.
Therefore, there is a need in the art for a process flow that creates a highly smooth surface of the intermetal dielectric layer that will serve as the substrate for the reflective pixel electrode.
A second process step leading to diminished reflectance of the pixel cell is the etchback of tungsten outside of the vias as shown in FIG. 3E. Highest intermetal dielectric layer 328 forms the substrate for the pixel metal electrode. Roughness in the surface of Tungsten in this underlying layer can promote roughness and a loss of reflectance of the electrode formed above.
Therefore, there is a need in the art for a process flow that removes Tungsten outside of the via while creating a highly smooth surface in the Tungsten remaining within the via.
A third process step leading to diminished reflectance of the pixel cell is utilization of a pixel electrode adhesion layer composed of Ti/TiN as shown in FIG. 3F. The character of the adhesion layer inherently affects the roughness of the overlying AlCu.
Therefore, there is a need in the art for a process flow that decreases the roughness of the adhesion layer and also permits further suppression of hillock formation.
A fourth process step leading to diminished reflectance of the pixel cell is the high temperature deposition of the pixel electrode layer shown in FIG. 3G. Deposition of the metal pixel electrode at high temperatures (&gt;200.degree. C.) leads to formation of AlCu having large grain sizes. Large grains of AlCu naturally increase the roughness of the pixel electrode layer, thereby diminishing its reflectance.
Therefore, there is a need in the art for a process flow that produces a metal pixel electrode layer having sufficiently small grains to promote reflectance of the pixel electrode.
A fifth process step leading to diminished reflectance of the pixel cell is stripping of the photoresist mask utilized to etch the pixel electrode layer as shown in FIG. 3I. Exposure of the smooth freshly deposited surface of the pixel electrode layer to harsh conditions of the solvent strip and asher clean necessary to remove the photoresist increase roughness in the pixel electrode surface, diminishing its reflectance.
Therefore, there is a need in the art for a process flow that prevents roughening of the surface of the pixel electrode due to the removal of the photoresist mask used to etch the pixel electrodes.
A sixth process step leading to diminished reflectance of the pixel cell is deposition of the passivation layer at a temperature substantially different from the temperature at which the metal pixel electrode layer was originally formed. Such a large temperature difference between these steps can lead to the formation of hillocks in the metal surface. Such hillocks are generated by shear forces exerted on the metal layer due to the differing coefficients of thermal expansion of the metal pixel electrode layer and the overlying dielectric layer. The hillocks render the surface of the pixel cell uneven, and thereby degrade reflectance.
Therefore, there is a need in the art for a process flow that prevents the formation of hillocks in the pixel electrode layer upon the formation of the overlying passivation layer due to the difference in temperature of formation of these layers.
Roughness in the pixel electrode surface caused by processing can adversely affect reflectance. However, it is also possible to enhance the reflectance of a pixel cell beyond that of a bare metal electrode by forming a reflective-enhancing coating (REC) over the pixel cell electrode. U.S. patent application Ser. No. 08/872,013, entitled "REFLECTANCE ENHANCING THIN FILM STACK" describes utilization of a reflective coating consisting of thin films of dielectric materials that increase reflectance. The thin dielectric films accomplish this result by promoting constructive interference of incident light reflected by the pixel electrode.
Therefore, there is a need in the art for a process flow that leads to the formation of a REC over the surface of a pixel electrode.